A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted electrode contact can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple stimulation contacts 112 on its surface that provide selective stimulation of the cochlea 104.
The electrode array 110 contains multiple lead wires embedded in a soft silicone body referred to as the electrode carrier. The electrode array 110 needs to be mechanically robust, and yet flexible and of small size to be inserted into the cochlea 104. The material of the electrode array 110 needs to be soft and flexible in order to minimize trauma to neural structures of the cochlea 104. But an electrode array 110 that is too floppy tends to buckle too easily so that the electrode array 110 cannot be inserted into the cochlea 104 up to the desired insertion depth.
U.S. Patent Publication 2010/0305676 (“Dadd,” incorporated herein by reference) describes winding the lead wires in the extra-cochlear segment of the electrode lead in a helical shape to make that portion of the electrode lead stronger. Dadd is quite clear that such a helical portion does not extend into the intra-cochlear electrode array which needs to be much more flexible than the extra-cochlear lead in order to minimize trauma to the cochlear tissues when the array is inserted.
U.S. Patent Publication 2010/0204768 (“Jolly,” incorporated herein by reference) describes winding the individual lead wires in the intra-cochlear electrode array in an elongated helical shape where each wire is separate and independent.
Electrode leads of active implantable medical devices including Middle Ear Implants (MEI's), Cochlear Implants (CI's), Brainstem Implants (BI's) and Vestibular Implants (VI's) need to be small in diameter but also they carry multiple lead wires. Electrode leads also need to be robust against external mechanical impacts, especially in locations where the electrode lead is placed on top of the skull bone only covered by the skin. In case of a mechanical impact on an unprotected electrode lead, the elastic silicone electrode carrier material is compressed and the electrode lead becomes temporarily locally thinner and elongated. Lead wires at the affected location experience local tensile forces and can even break. This is also the case for helically formed wires within a silicone electrode carrier since they are forced to expand nearly the same amount as the carrier material itself.
To deal with this problem, some implant designs arrange for the electrode lead to exits the implantable processor housing so that the electrode lead never lies superficially on top of bone. One disadvantage of such designs in the case of cochlear implants is that the implant housing must be placed in a very exactly defined position relative to the ear. For implant designs where the electrode lead emerges from the side of the implant housing, the surgeon is recommended to drill an electrode channel into the bone, which is time consuming so that not every surgeon follows the recommendations. Some electrode lead design include a rigid impact protector that surrounds the electrode lead, but that approach reduces the flexibility of the electrode lead which in turn makes the surgical implantation procedure more difficult. And in case of a mechanical impact in the area of the electrode lead, a rigid impact protector may protect the electrode from damage but also may cause trauma in the surrounding tissue when it is pressed against the protector.